A nerve cell can be excited in a number of different ways, but one direct method is to increase the electrical charge within the nerve, thus increasing the membrane potential inside the nerve with respect to the surrounding extracellular fluid. One class of devices that falls under the umbrella of Functional Electrical Stimulation (FES) realizes the excitation of the nerves by directly injecting charges into the nerves via electrodes which are either placed on the skin or in vivo next to the nerve group of interest. The electric fields necessary for the charge transfer are simply impressed via the wires of the electrodes.
FES is accomplished through a mechanism which involves a half-cell reaction. Electrons flow in wires and ions flow in the body. At the electro-electrolytic interface, a half-cell reaction occurs to accomplish the electron-ion interchange. Unless this half-cell reaction is maintained in the reversible regime, necrosis will resultxe2x80x94partially because of the oxidation of the half-cell reaction and partially because of the chemical imbalance accompanied by it.
The advantage of FES is that the stimulation can usually be accomplished from extremely small electrodes with very modest current and voltage levels. The disadvantage however, is that it involves half-cell reactions. Most rehabilitation programs using FES place the electrodes directly on the skin. A conductive gel or buffering solution must be in place between the electrodes and the skin surface. Long term excitation of nerve or muscle tissue is often accompanied by skin irritation due to the current concentration at the electrode/skin interface. This problem is especially aggravated when larger excitation levels are required for more complete stimulation or recruitment of the nerve group.
By contrast, magnetic stimulation realizes the electric fields necessary for the charge transfer by induction. Rapidly changing magnetic fields induce electric fields in the biological tissue; when properly oriented, and when the proper magnitude is achieved, the magnetically induced electric field accomplishes the same result as realized by FES, that of transferring charge directly into the nerve to be excited. When the localized membrane potential inside the nerve rises with respect to its normal negative ambient level of approximately xe2x88x9290 millivolts (this level being sensitive to the type of nerve and local pH of the surrounding tissue), the nerve xe2x80x9cfires.xe2x80x9d
The present invention is especially targeted at applications that are not suited for the use of implanted electrodes. The invention is designed for non-invasive external stimulation of selected nerve or nerve groups, particularly in certain applications. In these applications, which include incontinence and rehabilitation of muscle groups as well as potential weight loss treatment, the desired excitation levels using FES often fall outside of what might be considered comfortable limits. That is, the electrical current that ideally would be injected through the skin to excite the muscle groups of interest often leads to some skin irritation with time. The invention can also be used even in applications where this is not the case, as the use of gels and direct electrode/skin placement is inconvenient and is often resisted by the patient.
As opposed to FES, magnetic excitation has the attractive feature of not requiring electrode skin contact. Thus, stimulation can be achieved through the clothing that is being worn. This overcomes the objection of inconvenience and preserves the patient""s dignity. Secondly, because there is no direct contact, stronger excitation levels can be realized without undue additional skin irritation. A contribution offered by the present invention is the ability to achieve higher levels of focusing of the magnetic field and thus stimulation within the patient. Commensurate with this greater level of focusing comes some flexibility in the number of possible applications that might be targeted. Also accompanying the focusing is a higher level of power efficiency. Typically, the devices being designed by the methods outlined in this invention reduce the magnetic reluctance path by a factor of two. This reluctance reduction translates into a diminution of the current by the same factor and a fourfold reduction in power loss.
Magnetic stimulation of neurons has been heavily investigated over the last decade. Almost all magnetic stimulation work has been done in vivo. The bulk of the magnetic stimulation work has been in the area of brain stimulation. Cohen has been a rather large contributor to this field of research (See e.g., T. Kujirai, M. Sato, J. Rothwell, and L. G. Cohen, xe2x80x9cThe Effects of Transcranial Magnetic Stimulation on Median Nerve Somatosensory Evoked Potentialsxe2x80x9d,Journal of Clinical Neurophysiology and Electro Encephalography, Vol. 89, No. 4, 1993, pps. 227-234.) This work has been accompanied by various other research efforts including that of Davey, et al. (See, K. R. Davey, C. H. Cheng, C. M. Epstein xe2x80x9cAn Alloy-Core Electromagnet for Transcranial Brain Stimulationxe2x80x9d, Journal of Clinical Neurophysiology, Volume 6, Number 4, 1989, p.354); and that of Epstein, et al. (See, Charles Epstein, Daniel Schwartzberg, Kent Davey, and David Sudderth, xe2x80x9cLocalizing the Site of Magnetic Brain Stimulation in Humansxe2x80x9d, Neurology, Volume 40, April 1990, pps. 666-670). The bulk of all magnetic stimulation research attempts to fire nerves in the central nervous system.
The present invention differs in a number of respects from previous research efforts. First, the present invention has primary applicability to the peripheral nervous system, although it can be employed to stimulate nerves in the central nervous system as well. Second, and more importantly, the previous nerve stimulation work is dominated almost exclusively by air core coils of various shapes and sizes. The present invention is directed to the use of a magnetic core, more specifically a permeable core having a high field saturation, with the most preferred material being vanadium permendur. Among the air core stimulators are circles, ovals, figure eights, and D shaped coils. The coils are normally excited by a capacitive discharge into the winding of the core of these coils. This exponentially decaying field has a time constant typically in the neighborhood of 100 microseconds. Typical target values for the magnetic field peak happen to be near two Tesla. J. A. Cadwell is perhaps the leader among those who are now using and marketing these air core stimulators. Among his primary patents is U.S. Pat No. 4,940,453 entitled xe2x80x9cMethod and Apparatus for Magnetically Stimulating Neuronsxe2x80x9d Jul. 10, 1990. There are a number of power supplies all of which operate on a basic capacitive type discharge into a number of air core coils which are sold with his units. Various shaped coils are being explored at this time. One such coil is a cap shaped device which fits over the motor cortex (K. Krus, L. Gugino, W. Levy, J. Cadwell, and B. Roth xe2x80x9cThe use of a cap shaped coil for transcranial stimulation of the motor cortexxe2x80x9d, Journal of Neurophysiology, Volume 10, Number 3, 1993, pages 353-362).
Some efforts are being given to various circuits used to fire these air core coils. H. Eton and R. Fisher offer one such alternative in their patent xe2x80x9cMagnetic Nerve Stimulatorxe2x80x9d U.S. Pat. No. 5,066,272 Nov. 19, 1991. They suggest the use of two capacitorsxe2x80x94one to capacitively discharge into the coil of interest, and a second to recover the charge from the inductive energy resident in the coil. The circuit used in the present invention accomplishes the same objective with a single capacitor.
Some stimulation research is being performed on the peripheral nervous system (See e.g., Paul Maccabee, V. Amassian, L. Eberle, and R. Cracco, xe2x80x9cMagnetic Coil Stimulation of Straight and Bent Amphibian and Mammalian Peripheral Nerve in vitro: Locus of Excitation,xe2x80x9d Journal of Physiology, Volume 460, January 1993, pages 201-219.) The bulk of Maccabee""s work is however targeted for cranial excitation. The applications of the present invention focus on the peripheral nervous system although it can be used on the central nervous system, as well.
An object of the present invention is to provide a magnetic nerve stimulator for exciting nerves of the peripheral nervous system.
A further object of the present invention is to provide a magnetic nerve stimulator for non-invasive stimulation of nerves within the peripheral nervous system.
A further object of the present invention is to provide a nerve stimulator for the production of magnetic fields of significant depth and focusability to stimulate deep nerves within a human.
A further object of the present invention is to provide a magnetic nerve stimulator which can produce magnetic fields which can be focused on internal peripheral nerves to effect non-invasive nerve stimulation.
A further object of the present invention is to provide a magnetic nerve stimulator for the treatment of bladder and urinary disorders.
A further object of the present invention is to provide magnetic nerve stimulators for the treatment of incontinence.
A further object of the present invention is to provide a magnetic nerve stimulator for muscle rehabilitation and/or conditioning.
A further object of the present invention is to provide a magnetic nerve stimulator for use in assisting with weight loss.
Further objects of the invention will become apparent in connection with the disclosure provided herein.
To accomplish the objectives of the present invention, a magnetic nerve stimulator is provided herein which can be used to stimulate nerves without the need for surgery. Magnetic stimulation of peripheral nerves has the advantages of convenience and threshold variability over competing FES systems. An advance of the present invention over competing magnetic nerve stimulators is in the use of a highly saturable magnetic core, i.e. a permeable core of high field saturation, and in the design of the magnetic core stimulator itself.
In the preferred embodiment, the magnetic nerve stimulator is preferably constructed using a core of a magnetic or magnetizable material. A permeable material having a high field saturation is utilized, with the preferred core having a field saturation of at least 1.5 Tesla. Some suitable materials for the core include vanadium permendur, orthinol, metallic glasses (metglass), permalloy, supermalloy, powdered iron, and the silicon irons or silicon steels, in particular, 3% grain oriented steel (magnesil). Ferrite can also be used, although it is not preferred, due to the fact that it saturates at 0.5 T.
In accordance with the present invention, it is highly preferred that an open core be used. Toroidal cores are not preferred, as it is has been found that an open core can be more effectively utilized to focus the magnetic field produced by the stimulator, and as the suitability of toroidal cores have been found to be limited to invasive applications. By the term open core, an arc shaped core spanning an angle less than 360 degrees is intended. A 180 degree core is very convenient for using the material efficiently since two cores can be constructed from every mandrel. A core having a larger angle (e.g. 210-220 degrees) can also be used. These cores are more focussed, although they have a smaller penetration depth. Alternatively, cores of smaller or greater angles can be used in non-preferred embodiments.
In the current embodiments of the invention, it is an objective to xe2x80x9cfirexe2x80x9d a coil having approximately a 100 microsecond characteristic decay time, five (5) to fifty (50) times per second. The system must be reasonably efficient and reliable to fire at such a high repetition rate. Firing rates of 5 to 10 Hz are known to be effective for treating urinary stress incontinence using FES. Higher stimulation rates (e.g. fifty (50) Hz) have proved useful for treating irritative symptoms of urinary frequency and urgency. Sustained contractions occur above fifteen (15) Hz. As medical knowledge advances, various and as further research is conducted, other firing rates of higher or lower frequencies, or of particular excitation patterns, may prove useful in specific applications.
The exact stimulation frequency will be varied somewhat depending on the requirements of the application in need. Sometimes muscle groups will need to be excited for a five second period, followed by rest for a five second period and then be stimulated continuously for another five seconds and then rest again. While they are being stimulated, it is often desirable to have the muscle groups in a sustained contraction. This requirement dictates the necessity of continuing to pulse the cores at a repetition rate of 15 Hz. Because of the large currents involved during any given firing of the core, it is necessary to make the cores as efficient as possible. It is desirable to focus the magnetic field into the region targeted for stimulus to the exclusion of surrounding regions. The specially designed cores offered by this invention realize that focusability, whereas the air core coils used by the prior art do not.
With respect to the core configuration, the simplest core configuration of the present invention is that of a xe2x80x9cCxe2x80x9d shaped core. The span of the xe2x80x9cCxe2x80x9d must be carefully chosen; the span affects both the penetration depth and the magnitude of the field. Of additional importance is the construction of the core. The best cores are constructed from thin laminate materials having a high field saturation. A typical core can be wound using two mil stock of vanadium permendur. A long ribbon of such material is wound on a mandrel (e.g. a mandrel of wood or plastic) for the radius, thickness and depth desired. Each side of the ribbon is coated with a thin insulative coating to electrically isolate it from its neighbor. A generic core that might be used at various locations around the body can span an angle of approximately 180-220xc2x0. Once the ribbon has been wound on the mandrel to the desired dimensions, it is dipped in epoxy to freeze its position. Once the epoxy has cured, the mandrel is removed and the core cut for the span of angle desired. The cut will destroy the electrical isolation of adjacent laminations. Each cut must be finely ground so that it is smooth, and then a deep etch performed. The deep etch is performed by dipping each of the cut ends in an acid bath. This causes the cut ends to delaminate slightly, but maintains the electrical isolation of the laminations. Failure to perform this deep etch results in considerable eddy current loss and heating at the cut ends of the core. Following the deep etch, the ends are brushed with epoxy to maintain the shape and structural integrity of the core. The final step of the construction is to wind a coil of insulated wire about the core. A typical inductance for a core of this type is about 30 xcexcH. The present invention, however, may be practiced at other inductances or magnetic field strengths, as well.
In the simplest configuration, each core has only one winding. The winding is excited by an exponentially decaying pulse with a characteristic time of about 100 xcexcs. The actual signal has a ringing period of about that time within an envelope that is exponentially decaying so that only two to three cycles are ever witnessed by the coil current. The excitation is repeated on a period of about approximately 5-50 Hz. As stated above, the repetition cycle of these patterns will be varied according to the application. The circuit usually consists of a transformer which feeds into a full wave rectifier bridge. The bridge voltage charges the capacitor; the charge on the capacitor is triggered with a silicon control rectifier to drive current into the coil. The return charge coming back through the coil the second time is fed through the diode back into the capacitor to prepare the circuit for the second phase of excitation.
There are at least three important target applications for the present inventionxe2x80x94incontinence, muscle rehabilitation, and weight control treatment. For the treatment of incontinence, it is necessary to stimulate the pelvic floor muscles. Such a stimulation is achieved by concentrating and focusing magnetic flux directly up the vaginal cavity. One suitable core which is capable of realizing this objective is constructed by combining two individual xe2x80x9cCxe2x80x9d cores each spanning an angle of about 180xc2x0. The legs of the cores are brought together in a central region. The common central leg of the two xe2x80x9cCxe2x80x9d cores is wound by a coil and the return path for the flux is split between the two xe2x80x9cCxe2x80x9ds. The cores themselves fit proximally and distally under a chair which the patient sits on during treatment.
A second area of application is in the rehabilitation of muscles. The primary muscle groups targeted are the thigh, calf, biceps, and triceps. The geometry is similar for all these applications, and thus a cylindrical extension around the muscle is used. Although one solution for this problem is a simple xe2x80x9cCxe2x80x9d core and coil which is moved around by the discretion of the patient, an alternative stimulator resembles the tubular shape motors used in electromechanics to propel a secondary member down a tube. Here the geometry would necessarily require a hinged tubular shape having recesses or slots which would run azimuthally around the muscle group to be stimulated. The coils of the stimulator fits in these recesses or slots and the surrounding structure would again be a laminated vanadium composite. If the structure were fitted with two or three coils, they could be stimulated in a phased arrangement.
Such an excitation would have the effect of kneading the muscle tissue group along its longitudinal axis. This particular excitation pattern may be instrumental in more fully recruiting larger muscle groups such as the hamstring group in the leg. Full recruitment or stimulation of the nerve group would be advantageous to long term rehabilitation. Preliminary experiments with the device indicate that excitations at the frequencies mentioned accomplish exercise of the muscles at a higher efficiency and rate than could be accomplished through normal means.
Another area of application is that of assisting in weight loss management. As with muscle rehabilitation, one alternative is to simply use a handheld unit moved over multiple areas of the body. One group which can be particularly difficult to stimulate is the abdominal wall. An alternative method for realizing excitation of this group resembles a chest plate which can be secured to a patient or hinged to the side of a chair in which the patient sits. The chest plate contains a two or three phase arrangement of coils backed by the high field saturation cores constructed in the manner dictated above. The cores are spaced to drive the flux deeply within the abdominal muscle group. Both in muscle rehabilitation and in weight loss management, the phasing of the coils can be alternated with time to give the effect of a back and forth xe2x80x9ckneadingxe2x80x9d stimulation pattern. The rationale behind weight management is that the firing of these muscle groups requires the uptake of adenosine triphosphate; this energy expenditure is being artificially induced by the magnetic stimulator.
In summary, it is noted that there are a number of ways to more efficiently stimulate various muscle groups within the body. The key to these more efficient techniques revolves around using a thin laminate material of high magnetic field saturation to construct these cores and thereby drive and focus the flux into the regions desired. A simple xe2x80x9cCxe2x80x9d type core achieves a reluctance advantage of at least a factor of two over conventional cores. By using multiple cores connected at a center leg, a single focus site can be achieved with the return path disbursed in two or more areas so as to discourage excitation when the field is returned. In other applications, multiphased coils that actually enclose the tissue of interest can be excited so as to roll or knead muscle groups directionally with time. Certain wrapping applications may be more instrumental for higher recruitment of injured muscle groups.